Across Canada, impaired driving is driving with a blood alcohol concentration (BAC) above .08 or driving while impaired by alcohol or drugs. It is a serious offence to fail to give a breath sample when requested by police, with penalties the same as those for driving while impaired. A first offence conviction of impaired driving results in a criminal record, a minimum $1000 fine, and a driving prohibition for a minimum of one year.

Routine screening upon entry into Canada includes the question, “Have you ever been convicted of a crime?”  If you have been convicted of impaired driving - even if no collision was involved - you may be denied entrance.  Even with no other criminal violations. Think carefully.  Don't lie about any convictions, regardless of how 'trivial'.  This is especially true if  you're entering from the U.S.  Increased cooperation between Canada and U.S., as part of post-911 security measures, means that the border agent could already have access to criminal records. Lying/forgetting about a conviction could get you barred from entry into Canada for many years.  


A person with a conviction may be deemed rehabilitated and be eligible for entry after a certain period has expired from the completion of the sentence imposed  (which would include any driving suspension) on the conviction.   Depending on the offence, this period may be as short as 5 years or as long as 10 years. If a person cannot qualify for deemed rehabilitation, they may apply for individual criminal rehabilitation. A person may not apply for  criminal rehabilitation for 5 years following the original conviction (note the difference with deemed rehabilitation where the period begins with the completion of the sentence).  After this five-year waiting period (assuming the person has not been convicted of another offence) Americans (for example) can apply for “criminal rehabilitation” by submitting the following:
  1. An application form IMM 1444E
  2. A passport size photograph
  3. A copy of your passport data pages
  4. An FBI police certificate
  5. A state police certificate
  6. Copies of court documents indicating the charge, section of law violated, the verdict, and sentencing
  7. Proof of completed sentences, paid fines, court costs, ordered treatments, etc.
  8. Copies of the text of the law describing the offence.
  9. Detailed explanation of the circumstances surrounding the offence
  10. Three letters of reference from responsible citizens.
  11. A non-refundable processing fee of $180 USD
Further information can be found at Citizenship and Immigration Canada's webpages, Overcoming Criminal Inadmissibility and Overcoming Criminal Inadmissibility - Frequently Asked Questions.
It might be possible to get a temporary resident permit to enter Canada prior to rehabilitation, but this is up to the passport control officer's discretion and requires a $200 (Canadian) fee.  The temporary resident permit is meant to allow entry for exceptional circumstances, which would include reasons of national interest or on strong humanitarian or compassionate grounds. One more interesting twist on this involves transiting the Vancouver International Airport (and quite possibly any other Canadian airport).  When arriving on an international flight from the USA, you will be required to clear passport control, even if you are continuing to a connecting international flight with a destination outside of Canada.  If they find the DWI / DUI on your record they may send you to the immigrations screening and potentially a very long wait (they will not care if you miss your connecting flight).  

source: http://www.tripadvisor.com/Travel